Why is Olaf depicted with white hair, balding, if he's the same age as Lemony, about 40-ish?
Artistic license on Brett Helquist’s part. There weren’t as many details about Olaf’s actual age in “The Bad Beginning”. You can also blame alcoholism and poor hygiene for making people look much older than they actually are.
That being said, we also have to remember that this story is told from the spectrum of Lemony’s point of view (who hates Olaf), coupled with the Baudelaire orphans’ own perspective. So the descriptions given through the narration are HIGHLY subjective:
“Aren’t raspberries delicious?” he asked.
“They were my favorite berries when I was your age.” Violet tried to picture Count Olaf as a
youngster, but couldn’t. His shiny eyes, bony hands, and shadowy smile all
seemed to be things only adults possess. [The Bad Beginning, Chapter Six]
The interesting part is that we later learn (in “The Beatrice Letters”) that Lemony and Olaf were classmates during their teenage years. So Lemony definitely knows what Olaf looked like as a child, but chooses not to tell us.